As pointed out by Stefan0521 Sixt won't charge you for an additional driver and for automatic.
There is a simple reason why they don't have any fee for automatic. While you can book an automatic in the US there is no such possibility with Sixt. You can only request automatic but there is no 100% guarantee that you'll get one. In your case I'd give Sixt another call or email them and tell them that you need an automatic by any means and they'll do their best to give you one.
Enjoy your trip.
Cheers
totti